Plasma ejection system including breech and muzzle,theta-pinch coils



arch 10, 1970 R. TURNER PLASMA EJECTION SYSTEM INCLUDING BREECH AND MUZZLE, THETA-PINCH COILS Filed June 7, 196? Z MUZZLE con.

FIELD plasma formed Fig. 20

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INVENTOR Robert Turner 3,500,123 PLASMA EJECTION SYSTEM INCLUDING BREECH AND MUZZLE, THETA-PINCH COILS Robert Turner, Silver Spring, Md., assignor to the United States of America as represented by the Secretary of the Nav y Filed June 7, 1967, Ser. No. 645,098

Int. Cl. H01 1/50, 3/20, 7/24 U.S. Cl. 315-111 5 Claims ABSTRACT OF THE DISCLOSURE A theta-pinch plasma gun having two independent coils which ionizes and compresses an injected gas creates plasma when capacitor banks are discharged through the coils. An additional capacitor bank is discharged through one of the coils to eject the plasma from the gun.

BACKGROUND OF THE INVENTION The invention relates generally to plasma gun apparatus and method and more particularly to a plasma gun and method for generating and projecting plasma.

Production of plasma by inducing an electrodeless discharge in a low-pressure gas has been known in the art for over a decade. In practicing this technique, an electric field in the azimuthal (6) direction is induced in the gas by a rapidly varying external magnetic field. The azimuthal electric field causes the gas to break down and produces an azimuthal current in the gas. Interaction between the magnetic field and the current in the gas causes an implosion. Breakdown depends on the strength of the electric field and the gas pressure. The magnetic field used to create the plasma, a theta-pinch field, can be produced by discharging a capacitor through a heavy single turn coil wrapped about a vacuum enclosure in which gas has been injected. The coil and capacitor form a resonant circuit which causes a varying current to flow in the coil, thereby producing a varying magnetic field. In a standard thetapinch device, a plasma is produced in the coil during the first or second half cycle of oscillation. The plasma streams out both ends of the coil and no attempt is made to project the plasma. While this technique has the advantage of obtaining plasma without electrode contamination, it does not provide means for controlling the formed plasma, that is, the plasma cannot be projected in a single direction, nor can the shape of the pull? be regulated so as to concentrate the energy and minimize the long low-energy tail.

Prior attempts to create and contain plasma at the same place have been characterized by problems of instability in the plasma. One solution to the problem is to produce the plasma at one place and project it to another place for containment or further use. In order to accomplish this, some means for producing and directing the plasma in the desired direction is required. A modification of the single coil used in the theta-pinch technique has been suggested in which portion of the inside diameter of one end of the coil is replaced by a les conductive metal, such as stainless steel. The magnetic field then diffuses into the coil and weakens at that end of the coil causing the stronger field at the other end of the coil to exert a force on the plasma, thereby projecting it out the modified end of the coil. However, this technique does not achieve the desired result since the field diffuses much too slowly through the stainless steel for a satisfactory plasma gun and the plasma putt propagates out both ends of the coil substantially as before.

SUMMARY OF THE INVENTION Accordingly, one object of this invention is to provide an improved theta-pinch plasma gun.

3,500,123 Patented Mar. 10, 1970 Another object of this invention is to provide apparatus for controlling and projecting a plasma.

A further object of the present invention is to provide apparatus for producing a high-velocity putt of plasma having a high energy-density.

A still further object of the instant invention is to provide a method of generating and projecting plasma.

Briefly, in accordance with one embodiment of this invention, these and other objects are attained by providing in a plasma gun the combination of a vacuum enclosure with two coils. Both coils cooperate to set up a thetapinch magnetic field to produce plasma in the well known manner. An additional magnetic field is produced by one coil at substantially the time' the plasma is formed, which creates a stronger magnetic field at one end of the coil arrangement, thereby projecting the plasma out of the other end.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view showing the general structure of the plasma gun with its associated circuitry;

FIG. 2a is a graphical presentation of the magnetic field produced by the muzzle coil;

FIG. 2b is a graphical presentation of the magnetic field produced by the breech coil; and

FIG. 3 is a cross-sectional view showing an alternate embodiment of the breech and muzzle coils of the plasma gun.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings wherein like reference characters designate identical or corresponding parts throughout the several views and more particularly to FIG. 1 thereof wherein an embodiment of the plasma gun is shown consisting essentially of an elongate vacuum enclosure such as a tube 1 which may be formed of any suitable material of sufi'icient strength through which magnetic flux may pass relatively unaffected, such as glass.

An inlet 2 is provided for injecting a gas into tube 1. Coils 3 and 4, which may be designated as the breech coil and muzzle coil, respectively, surround tube 1 and are axially spaced apart. Ordinarily the coils are single turn and are formed of copper, aluminum, or any other suitable electrical conductor. A typical arrangement might use single turn aluminum coils, 3 cm. inside diameter, 3.5 cm. long, which are 1.5 cm. apart.

Electrical energy supplying devices such as capacitor banks 5, 6 and 7 are connectible to the coils by switching devices which are capable of handling large amounts of power, such as spark-gap switches 8, 9 and 10. More specifically, breech coil 3, capacitor bank 5, and sparkgap switch 8 are connected by electrical leads in a series relation. Upon closing spark-gap switch 8, the capacitor bank 5 and the breech coil form a series circuit. Capacitor bank 6 and spark-gap switch 9 are also connected by electrical leads to breech coil 3 in a series relation. Closing spark-gap switch 9 forms another series circuit consisting of the breech coil and capacitor bank 6. When spark-gap switches 8 and 9 are both closed, capacitor 'banks 5 and 6 are in parallel with each other. Capacitor bank 7 and spark-gap switch 10 are connected to muzzle coil 4 in a series relation by electrical leads. A series circuit of capacitor bank 7 and the muzzle coil is formed when spark-gap switch 10 is closed.

In operation, a gas (ordinarily hydrogen or helium) is pulsed into the breech end of the gun to a pressure of -200 microns. Spark-gap switches 8 and 10 are simultaneously closed, discharging capacitor banks 5 and 7 into the breech and muzzle coils, respectively. Since the completed electrical paths form L-C resonant circuits, oscillating currents, will flow in the breech and muzzle coils, respectively, thereby cooperating to produce a time varying magnetic field within the tube in the axial direction (a B field). It is to be understood that the coils are connected so as to both produce a field in the forward (breech coil to muzzle coil) direction during the second half-cycle of oscillation. An arrow emanating from the tube in FIG. 1 designates the forward direction.

By way of illustration a 2 pf. capacitor bank charged to 18 kv. produces a maximum magnetic field of about 18 kilogauss in approximately 0.5 sec. using coils of the type specified hereinbefore. With capacitor voltages above 20 kv. the plasma becomes unstable during the compression phase of formation, develops fiutes and expands quickly to the walls of the tube.

FIGURES 2a and 21) show the B fields produced by the muzzle and breech coils, respectively, as a function of time are shown by curves 11 and 12 on FIGS. 2a and 2b. Both fields are essentially of a damped sinusoidal form if no further capacitance is discharged into either coil. Plasma is formed by the theta-pinch field established by both coils, as described hereinbefore, during the second half-cycle of oscillation as shown in FIGURES 2a and 2b. During the second half-cycle of oscillation, sparkgap switch 9 is closed, discharging capacitor bank 6 into breech coil 3. For illustrative purposes, capacitor bank 6 may be 15 ,uf. charged to 13 kv. Curve 13 in FIGURE 2b shows the effect of connecting capacitor bank 6 to coil 3. The oscillation frequency of the field is decreased and the magnitude is increased substantially, thus ejecting the plasma from the gun. Since the plasma is essentially ejected from the gun by the completion of the first cycle of operation, the current flow in the coils after that time is of no concern. If desired, the gas may be preionized by discharging smaller capacitors (not shown) through the coils prior to the connection of main capacitors and 7. Needless to say, the plasma output is dependent on gas pulse and spark-gap switch timing and can be optimized in operation by one of ordinary skill in the art. The means for pulsing the gas, providing the proper gas pressure and initiating closure of the sparkgap switches at the proper times can be accomplished by techniques well known in the art and forms no part of the invention.

FIGURE 3 shows an alternate embodiment of breech and muzzle coils 3 and 4, respectively. The internal conical or tapered shape of the coils results in the production of a magnetic field which is stronger at the non-adjacent ends of the coils. The stronger field constitutes a magnetic mirror which tends to further contain the plasma before ejection. By way of example, a mirror-ratio (the ratio of the maximum to min. magnetic field) of 1.5:1 has been found suitable; too great a mirror-ratio results in plasma instability. A slight magnetic mirror effect oclcurs when using non-tapered coils due to the disruption of the magnetic field by the plasma itself.

The plasma-gun described has been found to produce hot, dense, high-velocity puffs of plasma having more than twice the energy of the breech coil used alone. Moreover, the puff produced does not have the long low-energy tail often found in prior art devices.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein. What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. Apparatus for generating and projecting plasma comprising, in combination,

a vacuum enclosure for receiving an injected gas, first and second axially spaced coil means surrounding said enclosure for establishing a theta-pinch magnetic field within said enclosure whereby the injected gas in ionized and compressed to form plasma, and

means for establishing an additional magnetic field within said enclosure subsequent to the establishment of said theta-pinch magnetic field whereby the formed plasma is projected, wherein said means for establishing said additional magnetic field is said second coil means.

2. Apparatus as described in claim 1, further characterized in that said means for establishing a theta-pinch magnetic field further comprises first and second electrical energy supplying means, and first switch means for connecting said first electrical energy supplying means to said first coil means and for connecting said second electrical energy supplying means to said second coil means at substantially the same time whereby a first magnetic field producing oscillating current flows through each of said coil means.

3. Apparatus as described in claim 2, further characterized in that said means for establishing an additional magnetic field further comprises third electrical energy supplying means and second switch means for connecting said third electrical energy supplying means to said second coil means subsequent to the connection of said first and second electrical energy supplying means to the first and second coil means, respectively, whereby the magnitude of the current flowing in the second coil means i increased and the oscillation frequency of the current is decreased.

4. Apparatus as described in claim 3, further charac terized in that said first and said second coil means are coils, respectively, and the inside diameter of said coils is tapered, being greater at their adjacent ends.

5. Apparatus as described in claim 3, wherein said second switch means connects said third electrical energy supply means to said second coil means during the second half cycle of oscillation of said first current.

References Cited UNITED STATES PATENTS 2,961,559 11/1960 Marshall 3 l363 2,997,436 8/1961 Little et al. 3l3-l6l X 3,270,236 8/1966 Koller et al. 313231 X 3,318,094 5/1967 Koller 60-202 3,321,664 5/1967 Phillips et al 313231 X JAMES W. LAWRENCE, Primary Examiner P. C. DEMEO, Assistant Examiner US. Cl. X.R. 

